Lignocellulosic biomass represents a renewable source of carbohydrate for biological conversion into fuels and chemicals and, as such, presents an attractive alternative to petroleum-based technology (Arntzen and Dale, 1999). It is recognized, however, that to reach its full potential, commodity production of ethanol from biomass will require high rates and efficiencies, simple processes, and inexpensive media (Ingram et al. 1998; Zhang & Greasham 1999).
Bacteria such as Escherichia coli have the native ability to metabolize all sugar constituents contained in lignocellulose. Early on, the qualities of environmental hardiness, broad substrate range, and ability to grow well in mineral salts media were recognized as important criteria that led to the selection of E. coli as a platform organism for metabolic engineering (Alterthum & Ingram 1989; Zhou et al. 2006a). Strain KO11 (ATCC 55124). Thus E. coli was engineered for ethanol production by integrating two Zymomonas mobilis genes (pdc, adhB) behind the pflB promoter of E. coli (Ohta et al. 1991). Despite the prototrophic nature of the E. coli strains, however, complex and costly nutrients were needed to rapidly and efficiently produce high ethanol titers using the KO11 strain (Asghari, et al. 1996; Martinez et al. 1999; Underwood et al. 2004; York & Ingram 1996).
To date, efforts to develop improved media and genetic modifications have been generally unsuccessful in eliminating the requirement for complex and costly nutrients, although betaine was found to be helpful (Underwood et al. 2004). Recently E. coli strain KO11 has been re-engineered to rapidly and efficiently ferment sugars to D (−)-lactate at high yields in mineral salts media (Zhou et al. 2006a and 2006b). However, to fully realize the potential of recombinant ethanologenic bacterial strains to serve as a source of ethanol, there is a need for new and improved strains of such bacteria that can efficiently produce ethanol while growing in inexpensive mineral media.